Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
Traditionally, scatterometry measurements are performed on targets consisting of thin films and/or repeated periodic structures. During device fabrication, these films and periodic structures typically represent the actual device geometry and material structure, or an intermediate design. As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers. In addition, the increasing number of parameters required to characterize complex structures (e.g., FinFETs), leads to increasing parameter correlation. As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements. In another example, opaque, high-k materials are increasingly employed in modern semiconductor structures. Optical radiation is often unable to penetrate layers constructed of these materials. As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.
In response to these challenges, more complex optical tools have been developed. For example, tools with multiple angles of illumination, shorter and broader ranges of illumination wavelengths, and more complete information acquisition from reflected signals (e.g., measuring multiple Mueller matrix elements in addition to the more conventional reflectivity or ellipsometric signals) have been developed. However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).
One of the industry responses to these recent challenges has been to incorporate x-ray metrology techniques such as x-ray photoelectron spectroscopy (XPS) and x-ray fluorescence (XRF). U.S. Patent Publication No. 2013/0077742 by Schueler et al. describes film thickness or dose measurements performed based on a combination of XPS and low energy XRF (LE-XRF). An x-ray illumination source provides x-ray illumination to the specimen and both XPS LE-XRF measurement signals are collected. This ensures that the XPS signal and the LE-XRF signal are both emitted from the same area of the sample under measurement.
The LE-XRF measurements described by Schueler et al. are limited by the low energy of the x-ray source that is required to maintain sufficient XPS photoelectron emission cross section (i.e., XPS signal). Low energy excitation limits the number of elemental lines which can be excited and most of the lines which are excited have significantly weaker fluorescence yield. This limits the effectiveness of LE-XRF measurements.
In one example, Schueler et al. describes an x-ray source that emits Aluminum Kα radiation having an energy level of 1.486 keV. Although this energy level is described as suitable for XPS, it has significant disadvantages for XRF. In this example, only fluorescence lines close to 1.486 keV or lower can be excited. For many materials of interest, the majority of fluorescence lines stimulated by this relatively low energy level are either L shell lines or M shell lines. FIG. 1 depicts a plot 10 of fluorescence yield as a function of atomic number. Fluorescence yield is defined as the probability that a core hole in a shell is filled by a radiative process, in competition with nonradiative processes. As illustrated in FIG. 1, the fluorescence yield is much lower for M shell and L shell lines then it is for K shell lines. In practice, the resulting XRF signal intensity of M shell and L shell line emission is orders of magnitude lower than K shell line emission. As a result, LE-XRF signals are typically orders of magnitude lower than high energy XRF signals. Moreover, as illustrated in FIG. 1, low fluorescence yield is especially problematic for low atomic number elements.
In another example, Schueler et al. describes an x-ray source having energy below the absorption edge (k-edge) of silicon (approximately 1.840 keV) as a compromise to obtain useable XPS and LE-XRF signals from the same x-ray source.
Future metrology applications present challenges for metrology due to increasingly small resolution requirements, multi-parameter correlation, increasingly complex geometric structures, and increasing use of opaque materials. Thus, methods and systems for improved measurements are desired.